Why 18 inch pipe is not 18 inches
Nominal Pipe Size is a label, not a measurement. NPS 18 pipe has an outside diameter of 18.000 inches, and it keeps that outside diameter in every schedule made. What the schedule number changes is the wall: at schedule 100 the wall is 1.156 inches, so the bore comes out at 15.688 inches.
That is the practical consequence. Fittings, flanges, supports and threading dies are sized to the outside diameter, so they fit any schedule of the same trade size. Flow, pressure drop and anything that has to pass through the pipe depend on the inside diameter, which changes with every schedule. Order by trade size and schedule; work your hydraulics from the bore.
Frequently asked questions
What is the wall thickness of 18 inch schedule 100 pipe?
1.156 inches (29.36 mm). The outside diameter is 18.000 inches (457.20 mm) whatever the schedule, so this wall leaves an inside diameter of 15.688 inches.
What is the inside diameter of 18 inch schedule 100 pipe?
15.688 inches (398.48 mm). Inside diameter is outside diameter minus twice the wall: 18.000 - 2 x 1.156 = 15.688 inches.
Is 18 inch pipe actually 18 inches?
No. NPS 18 is a trade size, not a measurement. The pipe measures 18.000 inches across the outside and 15.688 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
How much does 18 inch schedule 100 pipe weigh?
208.15 pounds per foot plain end (309.64 kg per metre), for carbon steel. Stainless of the same dimensions runs a little lighter, and the figure excludes coatings, linings and fittings.